1. Field of the Invention
This invention relates to lifting devices employed in solids-liquid sedimentation apparatus to raise and lower rotatable rake arms which engage settled solids within the tank and move the solids to a tank bottom outlet. More particularly, it provides a motorized cable-operated means to lift the rake drive unit with attached rakes above its normal operating position when it is desired to limit the torque applied to such unit and rakes.
2. Description of the Prior Art
Solids-liquid sedimentation apparatus which includes thickeners, clarifiers and the like employ a torque tube or shaft or a motor-driven ring gear or drivehead for driving one or more rake structures contained at a lower level within the sedimentation tank. In normal typical operations the rake(s) with associated blades move settled solids along a slightly conical bottom surface of the tank toward a central outlet where the settled solids are removed by gravity or pumping. A relatively high torque is imposed by the drive means during normal operation, but when an abnormal mass of sediment or an obstruction is encountered, the torque needed to drive the rake structure may be beyond the capability of the drive or the strength of the rake structure. When such an event occurs it is necessary to move the rake structure upwardly so its movement is not impeded by the heavy sediments or obstruction. This is normally done automatically in response to a torque sensor. Further, if power is lost or if it is necessary to shut down the rotation of the rakes for any reason, it is necessary to raise the rake structure above the resulting sediment layer so that the rakes are not "frozen" or "locked-in" the settled solids necessitating digging the solids out from around the rakes.
A relatively simple form of lift device is seen in U.S. Pat. No. 2,724,506 which includes lifting screw attached to a torque shaft which automatically raises the rakes by a camming action to a higher level where the rakes are no longer obstructed. The rakes may also be raised by rotation of the screw by a manually-operated hand wheel. U.S. Pat. No. 2,122,287 shows a similar hand-operated wheel for moving rakes upwardly.
The Bousman U.S. Pat. No. 2,727,628 shows a center column sedimentation device with a rotatable concentric cage therearound, supporting the rake structure. Raynor et al. U.S. Pat. No. 3,200,956 also shows a telescoping column lift device. Bousman U.S. Pat. No. 2,368,703 shows sedimentation apparatus including pulleys secured to the rake arms with cables extending from a rake blade through the pulleys to a winch for lowering or raising the arms. A similar pulley-cable-winch mechanism is utilized for raising an included cone scraper frame. U.S. Pat. No. 3,592,447 shows a pulley-cable-winch arrangement which raises or lowers a grain levelling apparatus from a fixed ceiling point.
At the present time most commercial sedimentation devices utilize one or more jack screws, rotation of which provides relative movement between a center column and a rake-supporting moving column. Such jack-screw devices have large load concentrations on the jack screw(s) and necessitate equalizion of loadings and rotational moments on each screw. Much space is taken up in the center column with the multiple screws and drives and the required bridge and walkway support columns. Further, jack screws inherently have low lift efficiencies of the order of 8-20%.